1. Field of the Invention
The present invention relates to a systems for multiplexing and transmitting a digital audio signal and a digital video signal, and more particularly to a signal multiplexing apparatus for multiplexing a digital audio signal and a digital video signal, and a signal separating apparatus for separating the digital audio signal and the digital video signal from the transmitted multiplexed signal.
2. Description of the Prior Art
Recently, as the method for transmitting digital video signals in a broadcasting station, the serial transmission system of transmitting digital data in a serial form has come to be used progressively. This transmission system is standardized as SMPTE (Society of Motion Picture and Television Engineers) 259M Standard. In this standard, in the synchronizing signal portion of the digital video signal, other signal than the video signal can be multiplexed and transmitted, and as an example thereof, the digital audio signal is multiplexed.
In such transmission apparatus, an audio signal entered in one line of a video signal is multiplexed and transmitted in the horizontal synchronizing period of the next or subsequent video signal. A buffer RAM (random access memory) is required for time axis correction of the audio signal in each of the transmitter and receiver. Generally, no audio signal is multiplexed in the line for changing over the video signal or in the next line to prevent influences of the changeover on the audio signal. In particular, in the composite video signal, audio signal is not multiplexed in the line containing the equalizing pulse, and the audio signals to be transmitted in this period are multiplexed together in a vertical synchronizing period, so that a buffer RAM of a sufficiently large capacity is required.
A structural example of such buffer RAM is disclosed in Japanese Laid-open Patent Application No. 4-313979. In this known art, a memory of two ports is used for time axis correction of audio signal. The writing address is compared with a value obtained by adding a specified value to the reading address, and the reading address is controlled so that the reading address added with the specified value may not overrun the writing address.
In this method, it is guaranteed only that the writing address is ahead of the reading address by the specified value. The mean value of the differences between the writing address and reading address is guaranteed more than the specified value, but does not become just the specified value. Therefore, the delay of the audio signal with respect to the video signal is not constant, and it varies depending on the initial state of the transmitting apparatus or the line from which the input video signal begins to be inputted. Fluctuation of the delay time of the audio signal is disadvantageous when the timings of the video and audio signals are adjusted to be the same by delaying the video signal with respect to the audio signal before transmission or after transmission.